The present invention relates to lithium niobate waveguide structures, particularly optical waveguide structures.
Lithium niobate waveguide devices provide, inter alia, a convenient way of implementing optical devices which require, for example, crossing, switching or combining of two or more waveguides. A lithium niobate device basically comprises a lithium niobate substrate (which may typically be of the order of 0.5-1 mm thick, 3-10 mm in breadth and 10-80 mm in length) with optical waveguides being provided in the substrate along selected paths of modified refractive index. The refractive index of the substrate may be modified along these paths, for example, by indiffusion of a titanium dopant, thereby raising the refractive index of the path above that of the adjacent substrate and creating an optical waveguide. The optical properties of such a waveguide can then be varied by the application of an electric field. This electro-optic effect enables the important use of lithium niobate devices as modulators and switches, for example. To take advantage of this electro-optic effect, suitable electrodes, commonly of aluminium or gold on chromium, must be deposited on the substrate.
In order to impose an electric field on the waveguide in a particular orientation, it may be desirable to provide an electrode directly above the waveguide itself. However, where an electrode is directly deposited on the waveguide, the evanescent optical field from the waveguide can be absorbed into the metallic layer, causing a high propagation loss in the tranverse magnetic TM mode.
To avoid this problem it is known to separate the metallic electrodes from the waveguide by interposing a layer of dielectric material, for example, silicon dioxide, of sufficient thickness between the waveguide and the electrode thus preventing the absorption loss. However, when more than one electrode is provided on a single dielectric layer, other deleterious effects, including leakage currents, for example, may be induced in the dielectric. Consequently, again, device performance can be severely degraded.